The RecA protein is a critical component in recombinational DNA repair, homologous genetic recombination and the cellular SOS response which occurs in response to DNA damage. The first RecA protein was purified from E. coli in 1965 by Clark and Margulies (Proc. Natl. Acad. Sci. (1965) 53:451) and has since been cloned and purified from many different organisms (Roca et al. Biochem. Molec. Bio. (1990) 25(6):415-456). Its role in genetic recombination is via a multi-step pathway involving the formation of a single-stranded nucleoprotein filament, DNA pairing and subsequent strand exchange to form heteroduplex DNA. RecA is directly involved in post-replication DNA repair and the induction of the SOS response by cleavage of the lexA repressor. RecA mediates a set of DNA strand exchange reactions in vitro by self-assembly into filaments, binding to both single-stranded (ss) and double-stranded (ds) DNA and hydrolysis of ATP.
The E. coli RecA gene and protein have been extensively characterized and numerous mutants have been isolated. Twenty-three recA-like proteins have been isolated from prokaryotes, both in closely related enteric bacteria as well as more distantly related prokaryotes such as cyanobacteria. These proteins exhibit amino acid sequence similarities to E. coli recA which range from 56% to 100% (Roca and Cox, 1990). The tertiary structure of these proteins has been conserved throughout the bacterial kingdom as measured by cross-reactivity with polyclonal antibodies raised against the E. coli K-12 recA protein. In addition, the uvsX protein of bacteriophage T4 exhibits significant structural and functional similarity to the E. coli protein (Story et al., (1993) Science, 259: 1892-1896). However, the T4 protein does not cross-react antigenically with antibodies raised against the E. coli recA protein and only has 23% identical amino acid residues (Fujisawa et al., (1985) Nucleic Acids Res., 13:7473-7481).
Recently, two eukaryotic recA analogues from S. cerevisiae, rad 51 and dmc1, have been isolated and characterized (Shinohara et al., (1992) Cell, 69:457-470; Bishop et al., (1992) Cell, 69:439-456). Although rad51 and dmc1 proteins share regions of strong amino acid similarity with E. coli RecA and will bind to ssDNA and dsDNA in the presence of ATP, they do not possess detectable ATPase or DNA strand exchange activities. In addition, based on amino acid sequence analysis of dmc1 and T4 UvsX, and crystallographic data from E. coli recA, the folded dmc1 and T4 UvsX proteins appear similar to the three dimensional structure of E. coli RecA .